Due to the nature of the lengthy design, fabrication and test processes, the method of developing integrated circuits (IC) for devices has in the past precluded the implementation of variable IC functions within a device. Any variation of device function due to variations in the chip has been due to the internal logic on the chip or due to a stored program on or outside of the chip. These logical functions are either deterministic, and thus reproducible by an unauthorized person or unit, or variable to such an extent that the performance of the chip is reduced, due to functional encoding, enciphering, path decoding, etc. The newer programmable gate arrays, programmable from a non-volatile memory on startup, can be reprogrammed as a defence against unauthorized possession of the device. These devices, being volatile, cannot be used in all applications, and suffer from the above mentioned performance losses. More important, however, is the fact that these variable devices represent merely the logical control inputs to the other secret devices. Heretofore there has been no adequate protection against gaining knowledge of the performance, structure and function of these main devices within the equipment.